When transmitting a signal between home appliances which are connected via a cable, or between semiconductor integrated circuits on a board that are connected via printed wiring, it is necessary to establish matching between the output impedance of the driving circuit and the impedance of the transmission line. The reason is that, if matching is not established, waveform distortion will occur due to reflection of the transmitted signal, thus hindering correct transmission of the signal. Furthermore, extra time is required until the signal reflection becomes subsided, thus making fast signal transmission difficult.
Conventionally, various methods for establishing matching between the output impedance of a driving circuit at the signal-transmitting end and the impedance of a transmission line have been known. Japanese Laid-Open Patent Publication No. 2003-8419, Japanese Laid-Open Patent Publication No. 10-261948, and Japanese Laid-Open Patent Publication No. 11-17518 describe techniques of establishing impedance matching when transmitting a signal between semiconductor integrated circuits. To describe the content of Japanese Laid-Open Patent Publication No. 2003-8419 as an example, separately in addition to a transmission line used for actual signal transmission, a reference transmission line is provided in loop fashion and terminated at the semiconductor integrated circuit which outputs a signal, the reference transmission line having equivalent characteristics to those of the transmission line. By utilizing the reference transmission line to establish matching between the output impedance of the driving circuit and the impedance of the transmission line, it is considered that impedance matching is also established with respect to the actual transmission line.
However, the conventional impedance matching techniques have various problems.
A first problem is that there may be an error between the characteristics of the transmission line which is used for transmitting an actual signal and the characteristics of the reference transmission line. In the case where the transmission line is a printed wiring board, naturally, the transmission line used for transmitting an actual signal and the reference transmission line are to be disposed at different positions on the printed wiring board. Even on the same printed wiring board, depending on the position, there may be variations in the characteristics (dielectric constant and the like) which determine impedance. Therefore, even if impedance matching is established by using the reference transmission line, optimal matching cannot be guaranteed with respect to the impedance of the actual transmission line. In other words, even by using the reference transmission line, it is difficult to determine the optimum impedance of the actual transmission line.
A second problem is that, since the reference transmission line must be provided separately in addition to the transmission line used for transmitting an actual signal, increases in area and volume may result. Especially in the case where there are plural signal transmission lines, and where precise impedance matching must be established, the same number or a close number of reference wiring patterns will have to be provided corresponding to the plural transmission lines, thus resulting in a great increase in wiring area. Such would be contrary to the trend for chip downsizing in the recent years, and therefore is impractical.
A third problem is that the aforementioned impedance matching techniques are not applicable to impedance matching between home appliance devices which are connected via a cable. In the case where a personal computer (PC) and a USB device are to be connected via a USB cable, for example, it will be impossible and impractical to provide a reference transmission line. Moreover, since the home appliance devices to be connected are diversified and it is impossible to establish impedance matching in advance, it will be necessary to establish impedance matching in a dynamic manner upon each connection. Especially in the case where signals are to be rapidly transmitted between home appliance devices, if matching between the impedance of a driving circuit and the impedance of a cable cannot be established, fast transmission may not be possible depending on the particular cable used, thus detracting from reliability.
An objective of the present invention is to, when transmitting a signal via a transmission line, establish matching between the output impedance of a driving circuit and the impedance of the transmission line in a dynamic manner, thus realizing fast signal transmission and improving transmission efficiency. Another objective of the present invention is to adapt the power consumption at the time of signal transmission to the transmission line, thus reducing power consumption to the bear minimum.